The present invention relates to extracorporeal systems for oxygenating and pumping blood during cardiac surgery. More specifically, the present invention relates to an integrated oxygenator and pump system having a rotating annular fiber bundle including flow-path lengthening baffles and internal recirculation paths.
Each year hundreds of thousands of people are afflicted with vascular diseases such as arteriosclerosis, that result in cardiac ischemia. For more than thirty years, such disease, especially of the coronary arteries, has been treated using open surgical procedures, such as coronary artery bypass grafting. During such bypass grafting procedures, a sternotomy is performed to gain access to the pericardial sac, the patient is put on cardiopulmonary bypass, and the heart is stopped using a cardioplegia solution.
Recently, the development of minimally invasive techniques for cardiac bypass grafting, for example, by Heartport, Inc., Redwood City, Calif., and CardioThoracic Systems, Inc., Cupertino, Calif., have placed a premium on reducing the size of equipment employed in the sterile field. Whereas open surgical techniques typically provide a relatively large surgical site that the surgeon views directly, minimally invasive techniques require the placement of endoscopes, video monitors, and various positioning systems for the instruments. These devices crowd the sterile field and can limit the surgeon""s ability to maneuver.
At the same time, however, the need to reduce priming volume of the oxygenator and pump, and the desire to reduce blood contact with non-native surfaces has increased interest in locating the oxygenator and pump as near as possible to the patient.
In recognition of the foregoing issues, some previously known cardiopulmonary systems have attempted to miniaturize and integrate certain components of cardiopulmonary systems. U.S. Pat. Nos. 5,266,265 and 5,270,005, both to Raible, describe an extracorporeal blood oxygenation system having an integrated blood reservoir, an oxygenator formed from a static array of hollow fibers, a heat exchanger, a pump and a pump motor that is controlled by cable connected to a control console.
The systems described in the foregoing patents employ relatively short flow paths that may lead to inadequate gas exchange, due to the development of laminar flow zones adjacent to the hollow fibers. U.S. Pat. No. 5,411,706 to Hubbard et al. describes one solution providing a longer flow path by recirculating blood through the fiber bundle at a higher flow rate than the rate at which blood is delivered to the patient. U.S. Pat. No. 3,674,440 to Kitrilakis and U.S. Pat. No. 3,841,837 to Kitrilakis et al. describe oxygenators wherein the gas transfer surfaces form an active element that stirs the blood to prevent the buildup of boundary layers around the gas transfer surfaces.
Makarewicz et al., xe2x80x9cNew Design for a Pumping Artificial Lung,xe2x80x9d ASAIO Journal, 42(5):M615-M619 (1996), describes an integrated pump/oxygenator having a hollow fiber bundle that is potted between an inlet gas manifold and an outlet gas manifold. The fiber bundle is rotated at high speed to provide pumping action, while oxygen flowing through the fiber bundle oxygenates the blood.
U.S. Pat. No. 5,830,370 to Maloney et al. describes a device having a fiber bundle mounted for rotation between a fixed central diffuser element and an outer wall of a housing. The fiber bundle is rotated at speeds sufficiently high to cause shear forces that induce turbulent flow within the blood. Within the fiber bundle, blood flows radially outward due to centrifugal force, and may have an insufficient residence time to provide adequate mass transfer at high bundle angular velocities.
Although the devices having rotating fiber bundles described in the foregoing references offer some desirable features, such as low priming volume and low surface area, it is unclear whether such devices can provide commercially desirable levels of oxygenation over a wide range of flow rates. In addition, such previously known devices may be subject to creation of stagnation and heat-buildup zones, such as near bearing surfaces.
Other patents have addressed the problem of reducing boundary layer impedance to mass and heat transfer in a stationary hollow fiber bundle by promoting radial cross-flow within the fiber bundle. For example, U.S. Pat. No. 5,352,361 to Prasad et al. describes a stationary fiber bundle having a plurality of axially symmetric baffles disposed along the shell side of a tube and shell heat/mass exchange system to promote radial cross-flow through the bundle. U.S. Pat. No. 5,169,530 to Schucker et al. describes a hollow fiber bundle having a plurality of radially disposed fluid impermeable baffles that divide the bundle into discrete sub-bundles. U.S. Pat. No. 4,220,535 to Leonard describes a tube and shell hollow fiber permeator having a partition in the shell portion of the bundle that enhances flow uniformity radial cross-flow in the resulting bundle zones.
While the foregoing patents describe the use of baffles to promote cross-flow in stationary fiber bundles, such baffling arrangements are uniformly absent in previously known rotating fiber bundle designs, where rotation of the fiber bundle itself provides adequate cross-flow through the fiber bundle.
In view of the foregoing, it would be desirable to provide an integrated blood oxygenator and pump having a rotating fiber bundle that provides compact size, low priming volume, low surface area and adequate oxygenation over a wide range of bundle angular velocities.
It also would be desirable to provide an integrated blood oxygenator and pump having a rotating fiber bundle, wherein the blood follows a tortuous path of increased length while flowing through the bundle, compared to having the blood pass outward along a radius of the bundle.
It further would be desirable to provide an integrated blood oxygenator and pump having a rotating fiber bundle, wherein a recirculation flow is provided near critical areas of the device to avoid heat build-up and stagnation.
It still further would be desirable to provide an integrated blood oxygenator and pump having a rotating fiber bundle and inter-bundle baffles that provide increased oxygen transfer compared to previously known rotating bundle designs.
In view of the foregoing, it is an object of the present invention to provide an integrated extracorporeal blood pump/oxygenator having a compact size, low priming volume and the ability to adequately oxygenate blood using a rotating fiber bundle that reduces boundary layer resistance to gas transfer and the formation of stagnation zones within the fiber bundle.
It is another object of the present invention to provide an integrated blood oxygenator and pump having a rotating fiber bundle, wherein the blood follows a tortuous path of increased length while flowing through the bundle, compared to having the blood pass outward along a radius of the bundle.
It is a further object of this invention to provide an integrated blood oxygenator and pump having a rotating fiber bundle, wherein a recirculation flow is provided near critical areas of the device to avoid heat build-up and stagnation.
It is yet another object of this invention to provide an integrated blood oxygenator and pump having a rotating fiber bundle and a plurality of baffles disposed within the bundle that provide increased oxygenation compared to previously known rotating bundle designs.
These and other objects of the invention are accomplished by providing an integrated blood pump/oxygenator, suitable for use within a sterile field, that has a low priming volume. In accordance with the principles of the present invention, the pump/oxygenator includes a rotating hollow fiber bundle assembly that both oxygenates the blood and develops additional pressure head, if desired, to pump the blood. The device further includes either (i) baffles for lengthening the flow path for blood passing through the fiber bundle and enhancing the oxygen transfer within the fiber bundle or (ii) means for recirculating blood to avoid the creation of stagnation or heat build-up zones.
In a preferred embodiment, the integrated blood pump/oxygenator of the present invention comprises an annular fiber bundle mounted for rotation within a housing, wherein deoxygenated blood is introduced into a central void of the bundle and passes outward through the bundle by centrifugal force. In accordance with the present invention, the annular bundle includes at one or more radial locations a plurality of baffles that extend over an arc of the circumference and that deflect the blood from following a straight-radially outward path. In addition, such baffles may have a radial dimension.
In accordance with another aspect of the present invention, appropriately sized gaps or pathways may be provided between the rotating annular fiber bundle and the housing to reduce the creation of heat-buildup or stagnation zones within the blood and to enhance recirculation and wash-out, thereby reducing the risk of blood trauma and platelet activation.